1. Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular to an improved stretcher fiber and module for use in a chirped pulse amplification system.
2. Background Art
Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort laser pulse up to the petawatt (1015 watt) level. A laser generates an output comprising a series of ultrashort pulses of relatively low power. These pulses are then fed into a stretcher module having a dispersion that causes the pulses to stretch out. The stretched pulse output is then fed into a high-power amplifier. The stretched, amplified pulses are then fed into a compressor module having a dispersion opposite to that of the stretcher module to produce a recompressed amplified pulse output. By stretching the pulses prior to amplification, it is possible to reduce the peak power of the pulses, and to avoid undesirable nonlinearities.
A compressor module is typically fabricated using free-space gratings and other components. The performance of a compressor module depends on a number of parameters, including the distance between gratings and the incidence angle. A compressor module design can be upgraded to meet the needs of a specific high-performance application by increasing the distance between gratings, decreasing the incidence angle, or both. Generally speaking, it is preferable to decrease the incidence angle. However, a decreased incidence angle results in an increased relative dispersion slope (RDS). Thus, the amount that the incidence angle can be decreased is limited by the availability of a stretcher module that can be matched to the compressor module.
Currently, in order to achieve a sufficiently high RDS, stretcher modules are fabricated using free-space diffraction gratings. However, for a number of reasons, it would be desirable to use an all-fiber-based solution. Although there have been efforts to create a fiber-based stretcher, these attempts have fallen short. In one case, 400 meters of standard low-cutoff single mode fiber was used as a stretcher. However, as a compressor typically has a positive RDS, and a standard low-cutoff single mode fiber typically has a negative RDS, no dispersion slope match is readily obtainable. To overcome this issue, a complicated balancing of the slope mismatch and nonlinearities in the amplifier fibers is required. In another case, a fiber with an RDS of 0.0053 nm−1 at 1050 nm was described. That fiber matches a grating compressor with a dispersion of β2=−1.6 ps2 (D=2.7 ps/nm) at 1050 nm, and is of limited utility. Thus, exists a need for an all fiber-based stretcher module that overcomes such drawbacks.